The invention relates to a method for producing alloyed steels, particularly stainless steels or steel prematerial for stainless steels, wherein in a first manufacturing step iron carriers are to a great extent decarburized and dephosphorized by means of oxygen and after removal of the slag resulting therefrom the melt is adjusted to the desired alloy and carbon content in a further manufacturing step after addition of alloy carriers by means of oxygen and inert gas, and a plant for carrying out the method.
A method of this type is known from EP-A2-0 229 586. Here, both manufacturing steps are carried out in one and the same oxygen-blowing converter. With this method, the amount of solid matter that can be charged for melting is very limited. With the oxygen-blowing converter, the maximum amounts of solid pig iron, alloying elements and scrap that can be charged are 20 wt. % of a charge. When intending to charge larger amounts of solids, one is forced to add expensive exothermic chemical heating agents, which involves the disadvantage of considerable amounts of slag (SiO.sub.2, Al.sub.2 O.sub.3, etc.). These considerable quantities of slag call for substantial additions of lime and, as a result, major yield losses in terms of iron, chromium, manganese, etc. incur.
According to EP-A2-0 229 586 the oxygen-blowing converter is provided with a bottom flushing means, in order to produce a turbulence of the molten bath. In the oxygen-blowing converter, this leads to high levels of chromium being oxidized into the slag, so that the economy of the known method is destroyed. The economically viable lower limit (at still acceptable loss of chromium into the slag) in respect of carbon content is at 0.2% C.
Moreover, the lowest carbon contents (e.g. less than 0.1% carbon) cannot be adjusted.
Up to now difficulties have been encountered when producing higher-alloyed steels in an electric furnace, particularly chromium-alloyed stainless steels, since extremely high levels of chromium slagging occur as decarburization is effected in the electric furnace. To avoid loss of chromium into the slag, it has been suggested to adjust temperatures of far over 1700.degree. C. during decarburization of the melt. As a consequence of these efforts, approximately 80% of stainless steels worldwide are made by converter methods.
The possibilities of the process route in the electric furnace--if desired in combination with a subsequent vacuum treatment--are very limited, in view of the charging materials that can be employed in an economical manner. Charging of phosphorus for example had to be limited to less than 0.030% and charging of carbon to less than f.i. 1%, since in the presence of chromium and due to reduction of the chrome oxide, dephosphorization is almost impossible and extensive decarburization in the electric furnace has not been successful so far because of the long periods involved and the high level of chromium slagging. In spite of the low carbon levels introduced when fusing low-P stainless alloyed scrap instead of major amounts of high-carbon ferro-chromium, premelts in electric furnaces conventionally have to be tapped at C contents of 0.5% to 1.2% and therefore must be subjected to a prolonged vacuum treatment in order to adjust the required low C contents, etc. The cost of this prolonged vacuum treatment is high and sequence casting is not possible.